| Abstract: | Rate constants for several reactions of inorganic radicals with inorganic anions in aqueous and aqueous/acetonitrile solutions have been measured as a function of temperature by laser flash photolysis. The reactions studied were (1) Cl2? + N3?, (2) Br2? + N3?, (3) Cl2? + SCN?, (4) Br2? + SCN?, (5) SO4? + Cl?, (6) SO4? + CO32?, and (7) N3? + I?. The rate constants were corrected for ionic strength and ranged from 106 to 109 L mol?1 s?1. The Arrhenius activation energies varied from 2 to 12 kJ mol?1 for the first 4 reactions, were higher for reaction 6, and negative for reaction 5. The pre-exponential factors also varied considerably with log A ranging from 5 to 14. The values of k298 decreased in most cases by more than an order of magnitude upon increasing the acetonitrile (ACN) fraction from 0 to 70%. For most reactions, this decrease in k298 was due to changes in log A with little regularity in the small changes observed in Ea. For reaction 7, k298 was practically unchanged due to compensating effects of the changes in Ea and log A with ACN mol fraction, giving an isokinetic relationship. An isokinetic relationship was also observed in the case of reaction 6; Ea and log A change in parallel while changing ACN mol fraction. Reaction 3 (Cl2? + SCN?) was also studied in water/t-butanol and water/acetic acid mixtures. Linear correlation was found between log k and the dielectric constant of the medium for water/ACN and water/t-BuOH but the lines for the two solvent mixtures had different slopes, suggesting specific solvation effects in addition to the primary solvent polarity effects. With water/acetic acid, k decreased and then increased upon addition of acetic acid. © 1993 John Wiley & Sons, Inc. |
